You just clamp it to the table, with the part you want to cut sticking off the table. Make sure the HDPE is clamped tightly. Turn the jigsaw on and then just cut the piece of the HDPE you want cut. You should probably buy work gloves to not cut your finger off

Also, I want to use an omniwheel as a caster for this robot. You know those small wheels on wheelchairs? And those metal attachments that connect the wheels to the wheelchair. What are those connections called? I want to get a small version of that, to attach the omniwheel to my robot so it can swivel.

I reused the same exact parts from this line following robot on the ERP. The rod was plastic and press fit between two plates with holes. You will also find an interactive CAD of it on that page, just rotate the robot to see the bottom.